TECHNICAL NOTES ON DRINKING-WATER, SANITATION AND HYGIENE IN EMERGENCIES
11
Measuring chlorine levels
in water supplies
As the quality of water can be seriously affected by a disaster
or an emergency, it is best practice to disinfect all emergency
water supplies. The most common way of disinfecting is with
chlorine. This technical note explains why disinfection is
important, why chlorine is used, how it works, how to test for
its presence and where and when to test.
Why should emergency
water supplies be
disinfected?
When disaster strikes a stable
community with access to
drinking-water of a certain quality,
their situation changes:
• Disasters often damage
existing water supplies leading
to contamination or further
contamination of the supply.
• People sometimes have to
move to new locations and
are forced to drink water from
new sources for which they
have no natural immunity to its
contamination.
• Disasters frequently affect
people’s physical and
psychological health, making
them more susceptible to
infection and disease.
It is important, therefore, that all
people affected by a disaster
are provided with access to safe
drinking-water. There is a wide
variety of methods for improving
the quality of drinking-water,
many of which are discussed in
Technical Notes 4 and 5. Most of
these treatment processes are
designed to prepare the water
for disinfection, which is the final
stage in the treatment process.
What is disinfection?
Many of the diseases that affect
traumatized communities are caused
by micro-organisms carried in
drinking-water. Hence the reference
to water-borne diseases. Disinfection
is the process of destroying these
organisms to prevent infection.
There are a number of methods of
disinfecting water, but chlorination
is by far the most common. Table
11.1 lists the advantages and
disadvantages of using chlorine for
disinfection.
due in part to their thick outer wall.
The process only works, however,
if the chlorine comes into direct
contact with the organisms. If the
water contains silt, the bacteria that
reside within it may not be reached
by the chlorine. Chlorine disinfects
water but does not purify it: there
are some contaminants it cannot
remove (see Box 11.1 overleaf).
How does chlorine work?
Chlorine takes time to kill
organisms. At temperatures of 18OC
and above, the chlorine should be in
contact with the water for at least 30
minutes. If the water is colder then
the contact time must be increased.
When chlorine is added to water,
it destroys the membrane of many
microorganisms and kills them.
However, it is ineffective against some
cysts, such as cryptosporidium, which
are resistant to chlorine disinfection
It is normal, therefore, to add
chlorine to water as it enters a
storage tank or a long delivery
pipeline to give the chemical time to
perform its disinfecting action before
it reaches the consumer.
Table 11.1. Advantages and disadvantages of using chlorine as a disinfectant
Advantages
Disadvantages
It comes in several forms: powder,
granules, tablets, liquid and gas.
It is a powerful oxidizing agent which must be handled
with care and breathing chlorine fumes must be avoided.
It is usually readily available in one form
or another and relatively inexpensive.
It does not effectively penetrate silt and organic particles
suspended in the water.
It dissolves easily in water.
It can give an unpleasant taste if slightly overdosed,
which can dissuade people from using the supply.
It provides residual disinfection (see Box
11.2).
It is effective against a wide
range of disease- causing microorganisms.
Its effectiveness against some organisms requires higher
concentrations of chlorine and longer contact times.
Is ineffective in removing cryptosporidium and where this
pathogen is a concern other methods should be used in
combination with chlorine (i.e. filtration).
Source: Adapted from Davis and Lambert (2002)
TECHNICAL NOTES ON DRINKING-WATER, SANITATION AND HYGIENE IN EMERGENCIES
Updated: July 2013
11.1
Measuring chlorine levels in water supplies
Box 11.1. Chlorine is not a
perfect solution
Although chlorine does not
destroy all micro-organisms,
it is still considered to be the
most effective emergency
disinfectant available because
the vast majority of organisms
are destroyed. Chlorine will not
remove chemical contaminants
from the water. Chemical
contamination is more difficult to
remove and requires specialist
knowledge and equipment.
Box 11.2. Residual protection
Most disinfection methods kill
micro-organisms effectively but
do not provide any protection
against recontamination further
along the supply system.
Chlorine has the advantage
of being both an effective
disinfectant and its residual can
protect the supply downstream
from the disinfection point.
So if water is tested and found to
contain some free chlorine, it proves
that the most dangerous organisms
in the water have been removed
and it is likely to be safe to drink.
This process is called measuring the
chlorine residual (see Figure 11.2).
Testing for chlorine
residual
The quickest and simplest method
for testing for chlorine residual is
the dpd (diethyl paraphenylene
diamine) indicator test, using a
comparator. A tablet of dpd is added
to a sample of water, colouring
it red. The strength of colour is
measured against standard colours
on a chart to determine the chlorine
concentration. The stronger the
colour, the higher the concentration
of chlorine in the water.
Several kits for analysing the
chlorine residual in water, such as
the one illustrated in Figure 11.2, are
available commercially. The kits are
small and portable.
When and where to
test water
Continuous chlorination is most
commonly used in piped water
supplies. Regular chlorination of
other water supplies is difficult and
usually reserved for disinfection
after repair and maintenance. It is
common to test for chlorine residual
at the following points:
• Just after the chlorine has been
added to the water to check
that the chlorination process is
working.
• At the outlet of the consumer
nearest to the chlorination point
to check that residual chlorine
levels are within acceptable
levels.
• At the furthest points in the
network where residual chlorine
levels are likely to be at their
lowest. If chlorine levels are
found to be below minimum
levels (see Box 11.3) it might be
necessary to add more chlorine
at an intermediate point in the
network.
The turbidity and the acidity (pH) of
the water have a significant effect
on the efficiency of chlorine as a
disinfectant. The turbidity should be
< 5NTU and the pH level between
7.2 and 6.8. See Technical Note 1
for advice on how to change the pH
level of water and measure turbidity.
Chlorine added
Initial chlorine concentration
added to water
Consult the references given under
“Further information” page 11.4 on
how to add chlorine to water.
Total chlorine
Remaining chlorine
concentration after chlorine
demand of water
Chlorine residual
When chlorine is added to water,
it will attack organic matter and
attempt to destroy it. If enough
chlorine is added, some will remain
in the water after all possible
organisms have been destroyed.
What is left is called free chlorine
(Figure 11.1). Free chlorine will
remain in the water until it too
dissipates or is used to destroy
new contamination.
11.2
Free chlorine
Concentration of chlorine
available for disinfection
Chlorine demand
Reactions with
organic
material, metals,
other
compounds
present in water
prior to disinfection
Combined chlorine
Concentration of chlorine
combined with nitrogen in the
water and unavailable for
disinfection
Figure 11.1. Chlorine addition
Source: Adapted from Chlorine Residual Testing Fact Sheet, CDC SWS Project.
TECHNICAL NOTES ON DRINKING-WATER, SANITATION AND HYGIENE IN EMERGENCIES
Measuring chlorine levels in water supplies
Step 1.
Step 2.
Place one tablet in the
test chamber (a) and
add a few drops of
the chlorinated water
supply under test.
Crush the tablet, then
fill chamber (a) with the
chlorinated water
supply under test.
a
a
Step 3.
Step 4.
Place more of the chlorinated
water supply under test
(without a tablet) in the
second chamber (b). This
is the blank control for
colour comparison.
The level of residual chlorine (R) in mg of chlorine per litre
(mg/l) is determined by comparing the colour of the water
supply under test in chamber (a) with the tablet added
with the standard colours on the vessel (chamber (b)).
Note that chamber (c) would be used if a higher chlorine
residual is to be used.
a
b
b
c
b
Figure 11.2. Steps in determining the chlorine residual in water using a comparator
The amount of chlorine residual
changes during the day and night.
Assuming the pipe network is under
pressure all the time (see Box 11.4)
there will tend to be more residual
chlorine in the system during the
day than at night. This is because
the water stays in the system for
longer at night (when demand
is lower) and so there is more
opportunity for the water to become
contaminated which reduces
the residual chlorine through
disinfection of the contaminants.
Chlorine residual should be checked
regularly. If the system is new or
has been rehabilitated then check
daily until you are sure that the
chlorination process is working
properly. After that, check at least
once a week.
TECHNICAL NOTES ON DRINKING-WATER, SANITATION AND HYGIENE IN EMERGENCIES
11.3
Measuring chlorine levels in water supplies
Box 11.3. Recommended
residual chlorine levels
The higher the residual chlorine
levels in the supply, the better
and longer the chemical will be
able to protect the system from
contamination. However, high
levels of chlorine make the water
smell and give it a bad taste,
which will discourage people
from drinking it.
For normal domestic use,
residual chlorine levels at the
point where the consumer
collects water should be
between 0.2 and 0.5 mg/l. The
higher level will be close to the
disinfection point and the lower
level at the far extremities of the
supply network.
Box 11.4. Chlorination and intermittent supplies
There is no point in chlorinating pipe networks if the water supply
is intermittent. All pipe systems leak and when the water supply is
switched off, the pressure will drop and contaminated water will enter the
system through the breaks in the pipe wall. No level of residual chlorine
acceptable to consumers will be able to deal with such high levels of
contamination. All intermittent water supplies should be assumed to be
contaminated and measures taken to disinfect water at the point of use.
A chorination checklist
•Chlorine needs at least 30 minutes contact time with water to disinfect
it. The best time to apply chlorine is after any other treatment process,
and before storage and use.
•Never apply chlorine before slow sand filtration or any other biological
process, as the chlorine will kill off the bacteria which assist treatment,
making the treatment ineffective.
•Never add any solid form of chlorine directly to a water supply, as it
will not mix and dissolve. Always make up as a paste first, mixing the
chlorine compound with a little water.
•Disinfection is only one defence against disease. Every effort should
be made to protect water sources from contamination, and to prevent
subsequent contamination during collection and storage.
•The correct procedure for applying a disinfectant to water should be
strictly adhered to, and water supplies should be monitored regularly
to ensure that they are free from bacteria. Otherwise, people may
be misled to believe that the water is safe to drink when, in fact, it is
hazardous to do so.
•The optimum chlorine residual in a small, communal water supply is in
the range of 0.2 to 0.5mg/l.
Further information
WHO (2011) Guidelines for drinking water quality,
4th ed., WHO, Geneva. http:www.who.int/water_
sanitation_health/publications/2011/dwq_guidelines/
en/
Davis J, Lambert R. (2002) Engineering in Emergencies
2nd edition, chapter 13. ITDG UK.
Centers for Disease Control and Prevention.Chlorine
residual testing fact sheet. CDC SWS Project
(Undated). http://www.cdc.gov/safewater/publications_
pages/chlorineresidual.pdf
Action Contre La Faim (2005) Water sanitation and hygiene
for populations at risk, chapter 11. Hermann Editeurs
Des Sciences et des Arts, Paris ISBN 2 7056 6499 8
Water, Sanitation,
Hygiene and Health Unit
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Prepared for WHO by WEDC. Author and Series Editor: Bob Reed.
Editorial contributions, design and illustrations by Rod Shaw
Line illustrations courtesy of WEDC / IFRC. Additional graphics by Ken Chatterton.
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11.4
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